1. Field of the Invention
This invention relates to a hydraulic control system for controlling line pressure to clutches and brakes for changing driving force transmission paths in an automatic transmission.
2. Description of the Prior Art
Typical conventional automatic transmissions for a vehicle comprise a torque converter and a multispeed transmission gear mechanism. The multispeed transmission gear mechanism includes a plurality of gear trains, and the driving force is transmitted through a desired gear train by selectively operating a brake and a clutch associated with the desired gear train. The brakes and clutches associated with the respective gear trains are operated by line pressure fed from a control valve. See Japanese Unexamined Patent Publication No. 58(1983)-196352, for example.
In such automatic transmissions, the gear-shifting is effected according to the throttle opening and the vehicle speed (the output speed of the transmission gear mechanism). That is, the throttle pressure corresponding to the throttle opening and the governor pressure corresponding to the vehicle speed are input into the control valve for controlling the brakes and clutches, and the gear-shifting is effected according to the throttle pressure and the governor pressure.
The automatic transmission vehicle is provided with a selector lever which is operated by the driver to select one of the driving ranges typically including R, N, D, 2 and 1. The automatic gear-shifting is effected within a limit determined by the driving range selected. For example, when D-range is selected, the gear-shifting is automatically effected between first to fourth according to the throttle opening and the vehicle speed. When 2-range is selected, the gear-shifting is automatically effected between second and third, and when 1-range is selected, the gear-shifting is automatically effected between first and second. There has been a demand in view of running performance of the vehicle that the shift pattern should differ depending on the selected range. For example, the gear-shifting between first and second in 1-range should be effected under conditions different from the gear-shifting between first and second in D-range.